“For I am not ashamed of the gospel, because it is the power of God that brings salvation to everyone who believes” Romans 1:16
The Gospel is the Bible’s message of Good News that Jesus Christ came to save people from their sin. God made mankind in His image, and though we all rebel and justly deserve punishment for rebelling against an infinitely great authority, He desires to have mercy on us. His mercy is conditional in the sense that we have to agree with what God says about our sinfulness, and believe Jesus Christ died in our place for our sin, and was raised to life again.
Contrary to what many people believe, Christianity is not a framework of rules like so many other religions (though many Christians unfortunately turn it into that). Christianity is about a relationship with a God who loves us enough to change us from the inside out, so that our very desires change as we submit to His rule in our hearts. This is the only way to truly do what Jesus said, “love your enemies and pray for those who persecute you.” That kind of love takes supernatural power. God wants to share that love and power, but His heart-transforming plan is contrary to everything we desire by nature and requires us to die to our own desires. Sadly, most people reject that relationship with Him because they are unwilling to let Him change their lives, and there are eternal consequences for that decision.
One of the best summaries of the Gospel that I have found is “Two Ways to Live” found here.